The subject of the invention is a parachute, whose canopy has a selected geometric porosity which is formed of holes, openings, and the like. In known parachutes, porosity is formed primarily by the inherent porosity of the parachute cloth. In more expensive special parachutes it is also known to build different vents into the canopy either for better stability or to lower opening shock.
Known parachute types and constructions however, cannot be directly constructed of non-porous plastic film or plastic coated cloth, which usually are weaker than parachute cloth. The reason for this is that, with known parachute constructions, it is impossible to overcome the unstability and high opening loads which follow from the use of non-porous material. It is known to utilize geometric porosity in certain cases either to improve stability, or in other cases, to lower the opening shock. A parachute construction, however, has not been suggested which would enable the use of non-porous and usually weaker material than regular parachute cloth, which simultaneously has good stability and a low enough opening shock to enable the use of the low strength material.
The opening shock for a regular parachute which is constructed of non-porous material can be four times the steady state drag force in the so-called infinite mass case. This situation exists under test conditions (in wind tunnels), but in normal use, under the so-called finite mass conditions, the mentioned opening shock ratio remarkably becomes even higher. This sets such high requirements for the material strength that they cannot be fulfilled with practical film thicknesses. Neither can enough stability be reached. Deflection angles of these parachutes reach 45.degree. and there exist strong oscillations in practice.
Textile cloth parachutes have a deflection angle of about 20.degree.-30.degree., the opening shock factor is about 2 and the parachute has a tendency to oscillate. Nylon cloth, however, is expensive and can cost many times the price of, for example, polyethylene or polypropylene films of suitable thickness. Therefore, the purpose of the invention is to find means to use non-porous material in parachutes having their opening shock and stability characteristics at least equal or even essentially better than the textile cloth parachutes and which parachutes of non-porous, inexpensive material would apply for example to cargo parachutes, brake chutes and ammunition chutes especially for one way or one time use.
As already mentioned, it is known to use geometric porosity to improve the strength and to lower the opening shock, deflection angle and oscillation of parachutes. As an example, one can mention such a parachute construction where the canopy is composed of elements having slots between each other. The often used ribbon parachute is of this type in which every gore is built of numerous ribbons with open space between each other. For example, from U.S. Pat. No. 3,218,007 there is known a combination of a regular parachute and a ribbon parachute, where the stability of the parachute is improved by making part of the gores of ribbons. However, due to the complicated manufacturing technique of the ribbon parachute one loses practically completely the advantage which would follow from the use of low cost material. From U.S. Pat. No. 3,284,032 there is known a parachute which has been stabilized by adding, to a regular solid flat parachute, a cylindrical band in such a manner that there is a gap between these two parts. From the known constructions this patentable construction would probably be best suited to be built also of plastic film. However, this known parachute has been built mainly of textile cloth. Fully satisfying results cannot be achieved even with this construction if manufactured of non-porous film. The gap between the disc and the band is too near the skirt of the parachute so that the air flowing out of the gap can not affect the overall air flow around the circular disc and the disc acts as an unstable smaller parachute which has a remarkably high opening shock. The cylindrical band however functions as a stabilizer and limits the overall deflection and oscillation to reasonably low values. The disc portion would not withstand the opening shock if it would be manufactured of non-porous film. In addition, the separate manufacture of the flat disc and the cylindrical band requires remarkably more work than if the whole canopy could be manufactured of uniform film, for example of uniform triangular gores.